The present invention relates to electrode units for electrically heating underground hydrocarbon deposits. More particularly, the invention relates to an electrode unit which, if hydrocarbons having a high viscosity and low fluidity are to be extracted, is used to feed electric current to the ground to heat the hydrocarbon deposit to increase the fluidity thereof.
The term "hydrocarbons" as herein used is intended to include petroleum, oil, bitumen contained in oil sand or tar sand and kerogen contained in oil shale. For simplification in description, these hydrocarbons will be referred to merely as "oil." Furthermore, the term "producing" or "production" as herein used is intended to mean extraction of fluid oil out of a well by self-spouting, pumping or fluid-transferring.
In the case where fluid oil is in the ground, a well is bored from the surface of the ground until it reaches the oil layer and fluid oil is extracted by spouting by the pressure of gas in the oil layer, by pumping fluid oil, or by injecting a liquid such as brine into one well under pressure so as to cause fluid oil to flow out of a second well. However, if the oil in the ground has a low fluidity, it is necessary to increase the fluidity of the oil prior to extraction through the well. In order to fluidize the oil, generally the oil is heated to decrease the viscosity thereof. Temperatures suitable for fluidizing oils depend on properties of the oil. In any event, it is necessary to heat the underground oil layer.
An oil layer can be heated by injecting hot water thereinto, by injecting steam at high temperature and at high pressure thereinto, by feeding electric current thereinto, by underground combustion in which an underground oil layer is ignited and then burnt by supplying air thereto, or by using explosives. The latter two methods are not practical because control thereof is considerably difficult.
For injecting hot water or steam at high temperature and high pressure, while an oil layer is heated to increase the fluidity of the oil, the oil fluidized can be spouted above the surface of the ground. However, if the oil layer includes a crack or a crevice having a high passage flow resistance, then the hot water or steam will flow through that part only. That is, the hot water or steam may not diffuse over the entire oil layer. Moreover, if an oil layer is hard and finely divided, the hot water or steam cannot diffuse therein, and accordingly it is difficult to heat the oil layer.
For heating an oil layer with electric current, a plurality of wells are bored in an oil layer, electrodes are disposed in the wells, and voltages are applied to the electrodes in the wells, so that the oil layer is heated through resistance heating. This technique is advantageous in that, even if an oil layer has cracks or is hard and finely divided, the oil layer can be heated in its entirety. However, it should be noted that the use of an additional device is required to extract the fluidized oil.
In order to increase the efficiency of production of oil, a method has been proposed in which, after an oil layer has been softened by heating by feeding electric current to an oil layer, the oil layer is maintained at an elevated temperature by injecting hot water or steam at high temperature and at high pressure to extract the fluidized oil. In order to efficiently heat the oil layer, it is essential to electrically insulate the electrode units in such a manner that the leakage of current to other than the oil layer is minimized. Furthermore, it is necessary that the electrode units be so designed that they cannot be damaged by the underground pressure, by steam used for heating, or the pressure or temperature of the injected hot water or steam.
In order to more concretely describe the electrode unit, the production of oil from oil sand will be described.
It has been confirmed that there are large deposits of oil or tar sand in the United States, Canada and Venezuela. The oil in the oil sand coexists with brine on the surface of a sand layer or between sand layers. Moreover, the oil in the deposits has a considerably high viscosity, and accordingly it is not fluid in the natural state. A part of the oil sand layer may be exposed in a canyon or on a river band. However, the larger part of the oil sand, having a thickness of several tens of meters, usually lies 200 to 500 m under the ground. Accordingly, from an economical point of view and from the standpoint of environmental protection, only limited amounts of oil sand can be dug from the ground and the oil separated therefrom. Therefore, it is a requirement to extract the oil directly from the underground deposit. If oil is produced from an oil sand layer lying at a short distance from the surface of the earth, the ground may cave in. Accordingly, it is desirable to extract oil only from oil sand layers lying more than 300 m underground.
FIG. 1 is an explanatory diagram illustrating a method of heating an oil sand layer with electric current. In FIG. 1, reference numerals 1 and 11 designate steel pipe casings, 2 and 12 insulators coupled to the casings 1 and 11, 3 and 13 electrodes coupled to the insulators 2 and 12, and 4 and 14 cables for supplying current to the electrodes 3 and 13. These elements form the electrode structure. Further in FIG. 1, reference numeral 5 designates a power source, 6 an oil sand layer, 7 current flowing between the electrodes 3 and 13, 8 the ground surface, 9 a layer above the oil sand layer (hereinafter referred to as "an overburden layer" when applicable), and 10 a layer beneath the oil sand layer (hereinafter referred to as "an oil sand lower layer").
When a voltage is applied across the electrodes 3 and 13 in the oil sand layer 6 through the cables 4 and 14 from the power source 5 located on the ground surface, current 7 flows between the electrodes 3 and 13 in an amount determined by the resistance of the oil sand layer 6, as a result of which the oil sand layer 6 is heated. In this operation, a part of the current 7 flows in the overburden layer 9 and the oil sand lower layer 10. However, since the insulators 2 and 12 are interposed between the electrodes 3 and 13, the amount of current flowing in the layers 9 and 10 is limited to a small value.
After the oil sand layer 6 has been heated sufficiently, the application of the voltage is suspended. Then, hot water or steam at high temperature and high pressure is injected into the oil sand layer 6 through one casing 1 of the electrode structure. As a result, hot water or steam together with oil flows out of the other casing 11. In general, the electrodes 3 and 13 have small holes therein in order to facilitate the flow of the hot water or steam.
FIG. 2 is a sectional view of a conventional electrode unit. In FIG. 2, reference numerals 3, 6 and 9 designate an electrode, an oil sand layer and an overburden layer, respectively, 15 a main conduit pipe assembly composed of a first conduit pipe 15a and a second conduit pipe 15b, 16 a first insulator disposed between the first and second conduit pipes 15a and 15b for insulating them from each other, 17 a second insulator which covers the first insulator 16 and surrounds the main conduit pipe assembly 15 near the first insulator 16, 18 a coupling through which the main conduit pipe assembly 15 is coupled to the electrode 3, 19 a partition member by which the electrode 3 is water-tightly separated from the main conduit pipe assembly 15, and 20 an electrical conductor which extends through the main conduit pipe assembly 15 and is connected through the partition member 15 to the electrode 3. Further in FIG. 2, reference numeral 21 designates an insulated oil supplying pipe which is arranged in the main conduit pipe assembly 15 and which opens near the partition member 19, 22 a water pipe which is also arranged in the main conduit pipe assembly 15 water-tightly penetrating the partition member and opening into the electrode 3, 23 cement filled in the gap between the main conduit pipe assembly 15 and a well 24 in which is inserted the electrode 3 with the cement being spread near the electrode, and 25 a blocking member for preventing salt water or hot water from rising through the gap between the cement 23 and the main conduit pipe assembly 15.
In heating the oil sand layer 6, brine is supplied into the water pipe 22 in the direction of the arrow A, and the salt water thus supplied flows through the holes 3a of the electrode 3 into the well as indicated by the arrows B thus filling the well. Then, insulating oil is supplied through the insulated oil supplying pipe 21 in the direction of the arrow C and is circulated in the direction of the arrow D. Under this condition, current is applied to heat the oil sand layer 6. After the oil sand layer has been heated for a certain period of time, the application of current is suspended, and instead of salt water, hot water is supplied through the water pipe 22 to heat the oil sand layer 6. Thereafter, similar to the case of FIG. 1, the oil sand layer is heated to cause oil to spout.
FIG. 3 is a cross sectional view of the above-described conventional electrode unit. As is apparent from FIG. 3, the electrical conductor 20, the insulated oil supplying pipe 21 and the water pipe 22 are not coaxial with the main conduit pipe assembly 15. Since the electrical conductor 20 is not coaxial with the main conduit pipe assembly 15, the impedance of the assembly 15 is higher than that which is provided when the conductor 20 is coaxial with the main conduit pipe assembly 15. In addition, as the insulated oil supplying pipe 22 and the water pipe 21 are arranged close to the electrical conductor 20, the impedance is further increased as a result of which the loss in current application is increased.
In the application of current to the oil sand layer 6, very little heat generated by the electrical conductor 20 is radiated, thereby leading to an increase in the temperature of the electrode structure. In addition, the conventional electrical conductor 20 is not flexible. Therefore, the electrical conductor 20 can be damaged due to the difference between the thermal expansion coefficients of the electrical conductor 20 and the main conduit pipe assembly 15 and it can be burnt as the temperature increases. Furthermore, the conventional electrode unit suffers from a drawback in that a temperature rise of elements adjacent to the electrode 3 cannot be prevented.
In the above-described conventional electrode unit, as is apparent from FIG. 3, the clearance between the water pipe 22 and the inner well of the main conduit pipe assembly 15 is small. The insulating oil is used to cool the electrical conductor. Therefore, when the oil sand layer 6 is heated by the hot water supplied through the water pipe, the insulating oil serves as a conductor for heat. Accordingly, a large amount of heat is conducted from the water pipe 22 through the insulating oil and the main conduit pipe assembly 15 into the overburden layer 9. In addition, it is necessary for the conventional electrode unit to have a device for maintaining the insulating oil at a low temperature. Thus, in the conventional electrode unit, the heat of the hot water is wasted by being conducted through the insulating oil and the main conduit pipe assembly into the ground, and furthermore a loss of heat occurs in cooling the insulating oil. That is, the conventional electrode unit has a low heating efficiency.
Moreover, the water pipe 22 involves a drawback in that, as in the case of the electrical conductor 20, it can easily be broken due to the difference between the thermal expansion coefficients of the water pipe 22 and the main conduit pipe assembly 15 when hot water is poured into the water pipe.
At a working site, the electrical conductor 20, the water pipe 22 and the insulated oil supplying pipe 21 are connected after which the main conduit pipe assembly 15 is connected. This operation is repeatedly carried out to assemble the electrode unit. Thus, the assembly of the electrode unit takes a great deal of time and labor.